The effects of methane addition to PRF95 (primary reference fuel with 95% volume of iso-octane and 5%
volume of n-heptane) on the fundamental combustion parameters are experimentally investigated in a
cylindrical combustion vessel using classical schlieren technique. In this study, methane is added with
three energy fractions of 25%, 50% and 75% to PRF95. The laminar flame propagation, Markstein length
and flame instability of dual fuels under different initial pressures and with different equivalence ratios,
especially under lean burn condition, are well studied. Spherical flames are experimentally investigated
at the initial temperature of 373 K and under the pressures of 2.5 bar, 5 bar and 10 bar. The equivalence
ratios vary with 0.8, 1.0 and 1.2. The stretched flame speeds are determined by outwardly spherical flame
method. The results show that at low initial pressures, the addition of methane to PRF95 increases the
stretched flame speeds with lean equivalence ratios while decreases it in rich region. Laminar flame of
methane-PRF95 mixtures burn faster than those of pure methane and PRF95 with equivalence ratio of
0.8 over the whole range of the initial pressures investigated, and this trend is more obvious at low pressure.
Comparing the data of 25% methane dual fuel (DF25) with that of base fuels with the equivalence
ratio of 0.8 and under the initial pressure of 2.5 bar, it can be seen that the flame speed of DF25 is 57%
faster than that of methane and 22% faster than that of PRF95. These results provide important theoretical
references to lean burn SI engine with methane-gasoline dual fuels under lean burn conditions.